1. Field of the Invention
The present invention relates to a frequency recovery apparatus and a mobile broadcast receiver using the frequency recovery apparatus.
2. Discussion of the Related Art
Generally, an Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme has been considered a specific format of multi-carrier transmission scheme in which a single data column is transmitted via a sub-carrier having a lower data transfer rate. If the OFDM scheme is employed for signal transmission, a transmission signal has very strong resistance to a frequency selective fading phenomenon or a narrowband interference phenomenon. Although a single carrier system may encounter a failure of overall links due to a single fade or interference phenomenon, only some carriers will be affected in a multi-carrier system. Therefore, a small number of erroneous sub-carriers may be corrected by a forward error correction scheme.
European-Digital Video Broadcasting (EP-DVB) has been established in Europe whereby many developers have conducted intensive research into a satellite broadcast scheme, a cable broadcast scheme, and a terrestrial digital broadcast scheme. Specifically, a Digital Video Broadcasting-Terrestrial (DVB-T) standard corresponding to the terrestrial digital broadcast scheme has selected the OFDM scheme as a transmission scheme.
A DVB-T transmission system performs mapping of data to be transmitted according to a modulation method when transmitting desired data using the OFDM scheme, controls the mapping result to pass through an Inverse Fast Fourier Transform (IFFT), inserts a guard interval into the IFFT result, and transmits the resultant data having the guard interval via a frequency domain. In other words, a single OFDM symbol is divided into a guard interval and a useful data interval. The guard interval data is equal to the duplication of data contained in the last part of the useful data interval. In this case, the guard interval insertion in individual OFDM symbols is adapted to prevent system performance deterioration caused by an Inter Symbol Interference (ISI) and a ghost.
Therefore, the DVB-T reception system performs a Fast Fourier Transform (FFT) of a received signal, such that it can perform signal demodulation in a general transmission scheme.
A frequency recovery function from among a plurality of synchronization components is adapted to allow a Radio Frequency (RF) carrier frequency of a transmitter to coincide with that of a receiver. A difference between the RF carrier frequencies of the transmitter and the receiver is called a frequency offset. The frequency offset occurs according to two serious influences associated with a reception signal.
First, the magnitude of a signal transmitted via individual sub-carriers is demodulated by the FFT, resulting in reduction of the signal magnitude. Second, Inter-Carrier Interference (ICI) occurs, such that orthogonality between sub-carriers cannot be maintained.
The OFDM scheme has a relatively very narrow frequency interval between sub-carriers when compared to a transmission band, such that it may cause a transmission signal to be greatly affected by a small frequency offset less than the sub-carrier interval. Therefore, a frequency recovery technology for use in the OFDM scheme is considered an important technology.
Generally, the OFDM scheme controls frequency recovery in two modes as opposed to a single carrier transmission scheme. The two modes are a fractional carrier recovery mode and an integral carrier recovery mode.
The integral carrier recovery mode estimates an integer multiple of a sub-carrier interval closest to an initial frequency offset and compensates for the estimated result. The fractional carrier recovery mode estimates a frequency offset less than half of a sub-carrier interval and compensates for the estimated result.
The fractional carrier recovery serves as a function for tracking a frequency offset having a predetermined size less than a half of a sub-carrier interval. Generally, it is well known in the art that a sub-carrier interval of 0.001 or less must be employed to neglect the influence of a frequency offset in a reception signal.
In this case, the fractional carrier recovery can be implemented in time and frequency domains. In more detail, the fractional carrier recovery can be implemented using a guard interval in the time domain located before an FFT unit and can also be implemented in the frequency domain located after the FFT unit using phase variation generated in a complex value received via a Continual Pilot (CP).
FIG. 1 is a block diagram illustrating a DVB-T reception system including a general frequency recovery device. An apparatus for compensating for the fractional carrier recovery in the time domain is shown in FIG. 1.
Referring to FIG. 1, a tuner 111 tunes an RF signal based on an OFDM scheme using an antenna, converts the RF signal into an Intermediate Frequency (IF) signal, and outputs the IF signal to an Analog-to-Digital (A/D) converter 112, resulting in a digitized IF signal. A signal digitized in the A/D converter 112 has only an In phase (I) component, and is applied to an I/Q divider 113, such that it is converted into a complex-component signal having a Quadrature (Q) component as well as the I component.
The digital complex signal is applied to a frequency-offset compensator 114. The frequency-offset compensator 114 multiplies the digital complex signal by the estimated frequency offset, such that it generates a baseband complex signal in which the frequency offset is corrected. It is assumed that the fractional carrier recovery has been performed in the time domain, such that the output signal of the frequency offset compensator 114 is transmitted to an FFT unit 115, and at the same time is transmitted to a fractional frequency offset estimator 117.
The FFT unit 115 removes guard interval data from the baseband complex signal generated from the frequency offset compensator 114, performs the FFT process on only useful interval data, and transmits a frequency-domain value to an equalizer 116 and an integral frequency offset estimator 118. The equalizer 116 receives a carrier distorted by a channel from the FFT-processed signal, and compensates for the received carrier.
The integral frequency offset estimator 118 estimates an integer part from among a rounded-off value of a relative frequency offset. The fractional frequency offset compensator 117 estimates a frequency offset of a decimal part serving as a difference in a rounded-off value and a relative frequency offset established prior to the rounding-off process. Specifically, the integral frequency offset estimator 118 reduces a frequency offset to ½ or less of the sub-carrier interval.
The fractional frequency offset estimator 117 fractionally adjusts the frequency offset to be zero. The frequency offset value of the decimal part estimated by the fractional frequency offset estimator 117 and the frequency offset value of the integer part estimated by the integral frequency offset estimator 118 are summed by a frequency offset estimation value mixer 119, such that an output signal of the frequency offset estimation value mixer 119 is transmitted to the frequency offset compensator 114. The frequency offset compensator 114 multiples the digitized complex signal by the estimated frequency offset value, such that it compensates for a frequency offset.
Mobile communication capable of allowing a user to view a TV via a receiver configured in the form of a mobile phone has been considered to be the most important TV-viewing method. The DVB has recently established a DVB-Handheld (DVB-H) scheme suitable for the above-mentioned mobile communication. Specifically, if the DVB-T scheme aims to implement a multi-channel or a high definition (HD, the DVB-H scheme aims to allow a user to satisfactorily view a Standard Definition (SD)—grade image of less than 2 Mbps or less while the user is in motion at high speed.
Two transmission methods have been widely used as the above-mentioned DVB-H transmission. In the first transmission method, a signal is multiplexed with an output signal of a DVB-T system and the multiplexed result is broadcast via the same transmitter. In the second transmission method, a signal is broadcast via a DVB-H dedicated transmitter in an independent channel.
However, the DVB-T scheme serving as the European terrestrial digital TV transmission scheme has been developed to receive TV signals suitable for a predetermined screen size of 12×40 inches. Therefore, if the system shown in FIG. 1 is applied to the DVB-H system, the DVB-H system is ineffective in power consumption because a mobile broadcast receiver is driven by a battery suitable for a screen size of 3×6 inches to receive TV signals.
Therefore, the OFDM scheme has many advantages over a single carrier system, but provides very weak resistance to a variety of synchronous errors such as a frequency offset. As a result, individual technologies required for designing the above-mentioned mobile broadcast receiver must be intensively studied by those skilled in the art.